PhD thesis - School of Informatics - University of Edinburgh

Chapter 5. Parsing English Inclusions 139
5.4 Parsing Experiments with a Hand-crafted Grammar
A second set of parsing experiments involve a German parser based on a hand-crafted
grammar, using the Lexical Functional Grammar (LFG) formalism, developed at the
University of Stuttgart. The nature of parsing German sentences containing English
inclusions with this monolingual parser will be analysed in detail. The aim is to de-
termine if inclusions pose as much difficulty as they do with a monolingual treebank-
induced parser and to test if additional knowledge about this language-mixing phe-
nomenon can be exploited to overcome this problem. Considering that the treebank-
induced parser sees at least some inclusions in the training data, although they are
sparse, a hand-written symbolic parser is expected to have even more difficulty in deal-
ing with English inclusions as it generally does not contain rules that handle foreign
material. Before focussing on the experiments, the parser is briefly introduced.
5.4.1 Parser
The Xerox Linguistic Environment (XLE) is the underlying parsing platform used in
the following set of experiments (John T. Maxwell and Kaplan, 1993). This platform
functions in conjunction with a hand-written large-scale LFG of German developed
by Butt et al. (2002) and improved, for example, by Dipper (2003), Rohrer and Forst
(2006) and Forst and Kaplan (2006). The version of the German grammar used here
contains 274 LFG style rules compiled into an automaton with 6,584 states and 22,241
arcs. Before parsing, the input is firstly tokenised and normalised. Subsequently,
string-based multi-word identification is carried out, followed by morphological analy-
sis, analysis guessing for unknown words and lexically-based multi-word identification
(Rohrer and Forst, 2006; Forst and Kaplan, 2006). Forst and Kaplan (2006) improved
the parsing coverage for this grammar from 68.3% to 73.4% on sentences 8,001 to
10,000 of the TIGER corpus by revising the integrated tokeniser.
The parser outputs Prolog-encoded constituent-structure (c-structure) and
functional-structure (f-structure) analyses for each sentence. These two representa-
tion levels are fundamental to the linguistic theory of LFG and encode the syntactic
properties of sentences. For in-depth introductions to LFG, see Falk (2001), Bresnan
(2001), Dalrymple (2001) and Dalrymple et al. (1995). While c-structures represent
the word order and phrasal grouping of a sentence in a tree, f-structures encode the